31.7 mm is actually 31.75 (1.25 inches from mountain bike standard). 31.8 mm is acutally 31.75. They are identical.I was wondering if someone knows if you have to purchase a Deda stem when using one of their bars. Their bars measure 31.7 not the usual 31.8 3T uses. Are they interchangeable with each other or is it best to use same make for both stem and bar?